1. FIELD OF THE INVENTION
This invention relates generally to downhole drilling motor apparatus of the positive displacement type, and more particularly to an air motor operated drilling tool coupled with a control valve in the power fluid supply line which prevents fluid supply to the drilling tool until a desired line pressure is obtained.
2. BRIEF DESCRIPTION OF THE PRIOR ART
It is known in the art of drilling well bores to utilize a drill pipe or drill string which has attached at its lower end a downhole drilling tool utilizing a positive displacement motor, the drive section of which is connected to, and rotates, a drill bit. In such apparatus a fluid, e.g., air, foam, or a relatively incompressible liquid, is forced down the drill string and on passing through the fluidoperated motor, causes rotation of a shaft ultimately connected to the drilling bit.
As the downhole drilling tool progress away from the drilling rig, more drill pipe is added between the motor (rotor/stator) and the drilling plateform or rig. As each joint of drill pipe is added to the drill string, air flow to the motor must be interrupted and the drill string emptied of air pressure. Once the connection of another joint of drill pipe is completed, the air flow to the downhole drilling tool can be initiated. Thus, the drill pipe is, in effect, forming an ever increasing expansion chamber as the drilling tool advances.
Expansion of the compressed air entering the empty drill string drastically reduces the initial air pressure and energy potential available to start the tool in operation. This energy potential builds up slowly because of a limited or fixed capacity for generating compressed air entering the lengthened drill pipe. The time required to fill the pipe increases as the length and/or diameter of pipe increases. Conventional downhole air motor drilling tools are difficult to start when they are at the bottom of the hole due in part to the compressibility of air and the flow restrictions of the drill cause the air pressure at the bottom of the string (adjacent the motor) to build up slowly. The aforementioned motors are also subject to leak paths at low pressure in the rotor/stator section which progressively open and allow air to bypass through the motor.
Another factor which causes difficult starting of the air motors is that they require a certain impulse of initial energy to initiate operation because of rotor/stator inertia, and internal friction and leakage. This may be further aggravated by ineffective lubrication or frost conditions from air expansion within the tool. In cold atmospheric conditions, an air motor may freeze moisture in the tight seal areas. Similar difficulties could occur from excessive drag on the drill bit. However, if pressure is applied to the motor in a substantially instantaneous manner, the motor will start and operate in a normal fashion. The present apparatus utilizes a control valve which allows an instantaneous, high-pressure blast of air to the down hole drilling tool motor to overcome the problem of difficult starting conditions such as those caused by long drill strings, a wet borehole, or freeze-up conditions.
There are several patents which disclose various valves having pressure operated mechanisms, none of which are used in the power fluid supply line of positive displacement drilling motors of downhole drilling tools, or utilize the present mechanism to prevent fluid supply to the pneumatic positive displacement motor of a downhole drilling tool until a desired line pressure is obtained.
Mason, U.S. Pat. No. 3,180,433 discloses a jar for drilling having a latch to prevent actuation of the tool until a predetermined velocity of the drive fluid is reached.
Jacobi, U.S. Pat. No. 2,276,979 Edman, U.S. Pat. No. 2,844,166, and Tennis, U.S. Pat. No. 2,848,014 disclose valves having pressure operated latch mechanisms, but not for controlling an air-pressure-operated earth boring tool.
Zinkiewicz, U.S. Pat. No. 3,137,483 discloses a ground burrowing device which utilizes a check valve rather than a pneumatic pressure operated valve. The Zinkiewicz valve operates by differential pressure or flow and would be opened by any small pressure difference between the front and back side of the valve. The valve is opened by application of pressure produces by movement of the hammer and not in response to air line pressure.
Kostylev et al, U.S. Pat. No. 4,629,008 discloses a percussive tool which utilizes a spring loaded valve but the valve is not responsive to air line pressure to prevent flow to the tool until air line pressure reaches a predetermined level.
The cited prior art and any other prior art known to applicants does not show an air operated downhole drilling tool having a control valve which prevents flow of air pressure to the tool until the air-line pressure reaches a predetermined level adjacent to the tool.